Recently, images of television, VTR or the like are enlarged. Development of the image displaying apparatus for displaying larger picture scenes is active. For the reason, larger size is difficult to make in the conventional direct viewing type television. Also, in the projection type image displaying apparatus using a CRT or a liquid crystal displaying apparatus such as liquid crystal panel (hereinafter referred to as TFT-LCD) driving by a thin film transistor liquid crystal panel, it becomes difficult to be higher in resolution for brighter operation. A projection type image displaying apparatus attracts public attention, using a light storing, or writing, optically addressed spatial light modulation (hereinafter referred to as OASLM) with a photoconductive layer and a modulating layer being combined (for example, Y. Tanaka and others, Japanese Journal of Applied Physics, Vol. 33 in 1994, Page 3469 through Page 3477 (Jpn. J. Appl. Phys. 33 (1994) pp. 3469-3477)).
Fundamental Art and its task
To understand the features of the present invention, the summary of the projection type image displaying apparatus using the most fundamental OASLM will be described in accordance with FIG. 17 through FIG. 27. The explanation in accordance with FIG. 17 through FIG. 27 indicates the contents experimented by us inventors, and is not prior art. FIG. 17 shows the basic construction of the OASLM. The OASLM 151 has construction where a photoconductive layer 4, a light reflection layer 5, and a light modulation 6 are held between glass substrates 2 and 2' with conductive transparent electrodes 3 and 3' being respectively provided on it. The light reflection layer 5 is divided into a plurality of pixels as shown. The photoconductive layer 4 is etched through the gaps between the pixels and a light absorbing layer 7 is formed therein.
When the write lights are inputted into the photoconductive layer 4 from the side of the glass substrate 2 of the OASLM 151, the voltage corresponding to the two-dimensional light intensity distribution of the image is applied upon the light modulation layer 6 and it switches in accordance with the voltage. As a result, reading lights which come from the side of the glass substrate 2' are modulated within the light modulation layer 6 and are outputted after the reflection by the light reflection layer 5. The light absorbing layer 7 prevents the reading lights from leaking to the photoconductive layer 4 from between the pixels of light reflection layer 5.
A projection type image displaying apparatus using the OASLM will be described in FIG. 18. The CRT, TFT-LCD and so on are used as an image source 8. The image inputting to the OASLM 151 is operated by focusing write lights 9 which are images of the image source 8 on the photoconductive layer 4 by the write lens 10. The reading lights 12 from the light source 11 are incident from the side of the light modulation layer 6 of the OASLM 151. The reading lights are modulated by the light modulation layer 6. After that, the lights have been reflected by the reflection layer 5, and are outputted through the light modulating layer 6 again. The output lights 13 are visualized through the visualizing means 14 and are enlargingly projected onto the screen 16 by the projection lens 15. The driving signal generating circuit 30 produces a driving signal S5 which drives the OASLM 151, and is produced from the vertical synchronous signal from the image source 8, to erase, and/or store images from the image source 8.
A liquid crystal material such as nematic type liquid crystal, ferroelectric liquid crystal and so on can be used as the light modulation layer 6, and the amorphous silicon of p-i-n structure can be used as the photoconductive layer 4. As a visualizing means 14, a polarization beam splitter can be used, and a metal hyride lamp, a xenon lamp or the like can be used as the light source 11.
In principle, it is possible to conduct a bright and higher-resolution image displaying operation by raising the output of the light source 11 and making the image of image source 8 into the OASLM higher in resolution in such a projection type image displaying apparatus.
A method of driving the OASLM 151 will be described hereinafter. FIG. 20 shows a basic driving signal wave form of the OASLM. The driving signal consists of a positive pulse (hereinafter referred to as erasing pulse) in synchronous relation with the vertical synchronous signal of the image and a negative pulse (hereinafter referred as writing pulse). The period where the writing pulse is applied to the OASLM is culled writing period, and the period where the erasing pulse is applied to the OASLM is called a erasing period. The OASLM reads in accordance with the light intensity inputted at a writing period so as to output the modulating light. An initializing operation is effected compulsorily in spite of the existence of the write light, during a erasing period, to remove the output to zero. When the respond-quick ferroelectric liquid crystal is used as the light modulating layer, the output becomes zero within several hundreds micro seconds after the impression of the erasing pulse.
FIG. 19 is a construction view of the driving signal generating circuit 30 for generating the driving signal of FIG. 20. In FIG. 19, the erasing voltage generating circuit 31 outputs the voltage of the erasing pulse, and the writing voltage generating circuit 32 outputs the voltage of the writing signal all the time. The pulse width controlling circuit 33, composed of a mono-stable multi-vibrator, normally outputs a voltage corresponding to the logic 1, and outputs (S4) a voltage corresponding to a logic 0 in a constant period after the rising edge input of the vertical synchronous signal S1. The switch circuit 34 and the resistance 35 feed the output voltage of the writing voltage generating circuit 32 when the output voltage of the pulse width controlling circuit 33 is a logic 1, and the output voltage of the erasing voltage generating circuit 31 when the signal is a logic 0, respectively to the output amplifier 36 (SS). The output amplifier 36 amplifies the input signal and outputs it.
In the evaluation of the image, the uniformity and the afterimage or the like are important parameters in addition to the brightness and the resolution. In the projection type image displaying apparatus using the OASLM shown in FIG. 17, it is impossible to satisfy simultaneously with three elements of high brightness, realizing of uniformity of the brightness and removing of afterimages.
The reasons for them will be described briefly with reference to the drawings. The brightness, uniformity of the brightness, and afterimage of the projection images are decided in a manner explained below. FIG. 21 is a time response wave form in a certain one point of image projected on the screen. The brightness of the image is decided by a product of an area of response wave form, i.e., the peak height and the ratio of the time (hereinafter referred to as time aperture factor) when the OASLM responds with respect to one field period (vertical scanning period). The height of the peak depends the brightness of the light source, the efficiency of a illuminating optical system (not shown), the modulating efficiency of the OASLM. The time response wave form depends upon a writing light intensity to be inputted for the writing period, and the timing when the writing starts. To raise the uniformity of the brightness, the time aperture factor of the OASLM has only to be equalized through the whole image with respect to the same writing light intensity. To remove the afterimages, the OASLM is required to be responded faithfully with respect to the input image to be inputted for each of one field period.
Difficulty in the simultaneous settlement of the task of the brightness, uniformity of the brightness, the afterimages in the projecting type image displaying apparatus will be described by way of an example where CRT is a writing source. In the case of CRT, a substrate emits its light immediately after an addressing by the electron beam and the light emitting operation is attenuated by its inherent time constant. Namely, for one point of the OASLM a writing light which is attenuated in terms of time is inputted.
The output of the OASLM in three points, such as upper portion, middle portion and lower portion, of the projection image is considered. Assume that the input signal is a white signal of NTSC (1 field period=about 16.7 milliseconds). In the OASLM, the erasing pulse is applied upon the whole face of the OASLM, synchronizing with the vertical synchronous signal. FIG. 22 indicates a driving signal wave form (FIG. 22 (a)) in the upper portion of the projection image, the time change of the writing light (FIG. 22 (b)), the response wave form of OASLM (FIG. 22 (c )). In the upper portion of projecting image shown in FIG. 22, the writing light is inputted (FIG. 22 (b )) within few milliseconds after the erasing operation has been conducted. The OASLM can respond almost 1 field period (FIG. 22 (c)), so the time aperture factor is high.
As shown in FIG. 23, the writing operation is effected (FIG. 23 (b)) at about 8 milliseconds after the erasing operation has been conducted in the projection image central portion. Although the OASLM starts to respond immediately after it, the erasing pulse is applied after the next vertical synchronous signal and the output of the OASLM becomes zero. Namely, the responding operation is conducted (the time aperture factor is about 50%) only for about as half as 16.7 milliseconds. In the lower portion of the projection image as shown in FIG. 24, the erasing operation is effected immediately after the writing operation starts. Therefore, the response period for 16.7 milliseconds is extremely shorter.
Namely, a timing in which the writing light is inputted is different among various places in the OASLM, but the erasing pulse is applied at the same time. Thus, the time aperture factor is different in space by the difference of the writing and the erasing timing, and the lower portion of the projecting image becomes darker. In this manner, the uniformity of the brightness of the projection image is spoiled considerably.
In order to realize the uniformity of the time aperture factor, (uniformity of the brightness of the projection image) (1) the storing light intensity of the image upper portion is lowered so as to be lower the time aperture factor of the whole image as that of lower portion, (2) the afterglow of the fluorescent substrate to be used in the CRT is made longer, i.e., the attenuation operation of the light emitting operation from the fluorescent substrate is made slower, (3) the response speed of the OASLM is made slower, and (4) the driving frequency is made higher (more than ten times) with respect to the field frequency. However, in the case of (1), the image becomes darker. In the methods of the (2) and (3), even if the brightness and the uniformity can be realized, it is impossible to reduce the afterimages. In the method of the (4), beating becomes conspicuous and sufficient uniformity cannot be obtained and the writing light intensity is necessary to increase considerably. Therefore, the resolution of the original image from the CRT is reduced.
For example, the brightness of the projection image is considered when the afterglow of the fluorescent substrate of the CRT is made longer. The response of the projection image of the central portion is shown in FIG. 25. The erasing operation is conducted once after about 8 milliseconds from the light emission. When the afterglow of the fluorescent substrate is long, so the writing light which can respond the OASLM sufficiently is given, even at the time when the next writing period has been applied. So, the OASLM responds again. It is possible to obtain high time aperture factor as approximately much as that of the upper portion as shown in FIG. 22. This is similar as that even in the lower portion of the projection image. Namely, it is possible in principle to make the time aperture factor the same as that in all the location of the projection image by adjusting the strength of the light to be written.
But when specific signal (for example, the black signal) of the image which is different from that of the prior field is inputted continuously, the OASLM cannot be responded faithfully with respect to the change in the signal. A further description will be given in connection with FIG. 26 showing the response wave form of the projection image central portion. The afterglow of the florescent substrate of the CRT is long as shown in FIG. 26 (b). Therefore, the responding operation is conducted again (FIG. 26 (c)) after the OASLM has been erased by the erasing pulse at about 8 milliseconds after the response. As the next erasing operation is conducted after 16.7 milliseconds, the OASLM remains responded even in the time where the black should be originally displayed. Namely, the afterimage of about 8 milliseconds is caused. As a result, when a flicker signal consists of repeat of the white and black signal for each 1 field period is inputted, the output of the OASLM is repeated in half tone as shown in FIG. 27.
Such afterimages are considerably reduced in the resolution degree of the moving image, so as to give large different feeling to an observer. For example, the moving object of the image looks leaving a trail. The same afterimage can be observed even by the OASLM with slow response of the light modulation larger. As above, in the conventional projection type image displaying apparatus using the OASLM of FIG. 17, it is impossible to simultaneously settle the problems of the brightness, uniformity and afterimage.
An object of the invention is to provide a projecting type image displaying apparatus capable of image display which is brighter, higher in uniformity and extremely reduced in afterimage.